With the development of an information-oriented society, the requirement for high speed processing of a large capacity of information increases more and more in recent years. In such a situation, the optical communication system had been established with the rapid advancement of a series of techniques such as a high purification technique of silica glass, a transmission loss lowering technique, a technique for making a high output, and long life semiconductor laser.
On the other hand, compared with the silica optical fiber occupying the mainstream of the optical communication, the synthetic resin optical fiber has the disadvantage, i.e., a relatively large transmission loss, but it has the following advantages: (1) it is possible to give a large aperture thereto; (2) it is possible to increase the numerical aperture(NA); and (3) it is flexible and easy to handle. Therefore, it is expected to apply this synthetic resin optical fiber to the field of decoration such as a display and the like, and the shortdistance communication as in office and factory automations, communication in an automobile, etc.
A conventional synthetic resin optical fiber comprises a core consisting of a very transparent resin material and a cladding or clad layer consisting of a synthetic resin having a refractive index lower than that of the core material. However, if such an optical fiber is used as it is, the external light will reach to the core to cause increased noise due to the transparent cladding. In order to cut off the external light and to improve the mechanical property against an external, side pressure, etc., a protective layer containing a light interruption substance is provided around the cladding. In general, this protective layer is formed by extrusion molding process, etc. after forming the structure comprising the core and the cladding.
The provision of such a protective layer results in a complicated fabrication process, and thus high production cost, and the problem which the transmission characteristics of the optical fiber is influenced by the stress developed upon heating and the like due to the differences in the coefficient of thermal expansion and the modulus of elasticity of respective constituent materials thereof.
Such a synthetic resin optical fiber is described in Japanese Patent Laid Open No. 190903/82. The synthetic resin optical fiber comprises a core, and a cladding of adhesives, containing a light absorption substance such as carbon etc. and having the same refractive index as that of the core, which is provided on an outer peripheral surface of the core, so that light leaked out of the core is absorbed in the cladding whereby the synthetic resin optical fiber is applied to a transmission of image data. In fact, however, light is leaked from the core to the cladding when it is bent, although external light is interrupted in the presence of the cladding. For this reason, it is difficult that the synthetic resin optical fiber is practically applied to an optical communication.
Another synthetic resin optical fiber is described in Japanese Patent Laid Open No. 34333/74. The synthetic resin optical fiber comprises a core and a cladding provided around the core, both of which are colored by containing organic dye thereby providing a wavelength filtering property. As a result, light of a specific wavelength is only transmitted through the synthetic resin optical fiber. However, it is also difficult that the synthetic resin optical fiber is practically applied to an optical communication because light transmission characteristic is largely deteriorated due to the dye contained in the core.